The present application relates to a positive electrode and a nonaqueous electrolyte secondary cell, particularly to a positive electrode active material, a positive electrode and a nonaqueous electrolyte cell with which a high capacity and excellent charge-discharge cycle characteristics are secured and in which deterioration during use in a high-temperature environment is little.
Owing to the remarkable progress of mobile electronics technology in recent years, such electronic apparatuses as cellular phones and notebook size computers have come to be recognized as fundamental technology for supporting the advanced information society. In addition, research and development concerning enhancement of functions of these electronic apparatuses have been made vigorously, and the electric power consumption by each of the electronic apparatuses has also been steadily increasing accordingly. On the other hand, these electronic apparatuses are required to be driven for a prolonged time, and secondary cells and batteries as driving power sources for them have necessarily been desired to show an enhanced energy density. Besides, because of environmental concern, the secondary cells and batteries have also been desired to have a prolonged cycle life.
From the viewpoint of the volume, mass, etc. of the cell or battery incorporated in an electronic apparatus, a higher energy density of the cell or battery is more desirable. At present, lithium ion secondary cells or batteries have come to be incorporated in most of these apparatuses, because of their excellent energy density.
In lithium ion secondary cells, normally, lithium cobaltate is used for the positive electrode, while a carbon material is used for the negative electrode, and the operating voltage is set in the range of 4.2 to 2.5 V. The fact that the terminal voltage in the electric cell can be raised to 4.2 V is largely owing to the excellent electrochemical stability of the nonaqueous electrolyte material, separator and the like used in the cell.
Since the lithium ion secondary cells have a high charging voltage as compared with those of other cell systems, however, the lithium ion secondary cells have a problem in that their capacity would be deteriorated in a use mode in which the cells are left to stand for a long time in the charged state, resulting in a shortened battery life. In addition, where a lithium ion secondary cell is used in a high-temperature environment, a rise in internal resistance would proceeds, probably making it impossible to take out a sufficient capacity.
In order to solve the above-mentioned problems, Japanese Patent No. 3197763 (hereinafter referred to as Patent Document 1) describes addition of a metallic salt or hydroxide to a positive electrode. Besides, Japanese Patent Laid-open No. Hei 5-47383 (hereinafter referred to as Patent Document 2) describes coating of surfaces of lithium cobaltate (LiCoO2) with phosphorus. Further, Japanese Patent No. 3172388 (hereinafter referred to as Patent Document 3) shows coating of a positive electrode active material or a surface of a positive electrode with a metallic oxide.
Japanese Patent Laid-open No. Hei 7-235292 (hereinafter referred to as Patent Document 4) describes an approach in which a lithium-transition metal compound oxide is uniformly deposited on surfaces of particles or is diffused into the particles from the surfaces. Japanese Patent Laid-open No. 2001-256979 (hereinafter referred to as Patent Document 5) describes a positive electrode active material in which lumps of a metallic oxide are deposited on a metallic oxide layer. Japanese Patent Laid-open No. 2002-164053 (hereinafter referred to as Patent Document 6) discloses a positive electrode active material layer in which surfaces of particles containing a lithium compound are provided thereon with at least one surface treatment layer containing at least two coating elements. Japanese Patent Laid-open No. 2003-221235 (hereinafter referred to as Patent Document 9) describes an approach in which the energy value of fluorine at surfaces of particles as measured by XPS (X-ray Photoelectron Spectroscopy) is specified.
Japanese Patent Nos. 3157413 and 3141858 (hereinafter referred to as Patent Documents 7 and 8) disclose a positive electrode active material in which a coating having a metallic fluoride is formed on surfaces of particles, and coating of particles with a crystalline metallic fluoride.